1. Field of the Invention
The present invention relates to a process for electrolytically producing amalgam from metal salt.
2. Background of the Invention
Amalgams are alloys of mercury with alkali metals, alkaline-earth metals or transition metals. They are widely used in industry. In dental technology, tin/copper/noble-metal amalgams are used as tooth fillings. Gold amalgam and silver amalgam can be used to purify mercury-containing gases and liquids. The use of alkali-metal amalgams as reducing agents in the chemical industry, for example for producing alkali-metal alcoholates, is well-known. Sodium amalgam is an important intermediate in chlor-alkali electrolysis. The amounts of amalgam used on a large industrial scale for reactions are normally obtained by this process. The desired amalgam is obtainable at a mercury cathode by electrolysis. Alkali-metal amalgams are usually produced as intermediates in the amalgam process of chlor-alkali electrolysis. In this process, aqueous sodium chloride solution is decomposed by electrical energy to form sodium hydroxide solution, chlorine and hydrogen: EQU 2NaCl+2H.sub.2 O.fwdarw.2NaOH+H.sub.2 +Cl.sub.2
The anode product chlorine is separated from the cathode product amalgam. The amalgam flows in the recirculating system via a so-called decomposer connected downstream and is electrolytically further decomposed in aqueous solution in the latter to form sodium hydroxide solution with simultaneous evolution of hydrogen. Mercury is used as cathode and a graphite anode or an activated titanium anode is usually used as anode.
The use of chlorine in industry is restricted because of a potential safety and health risk of chlorine gas and of chlorinated reaction products, such as, for example, CFCs, vinyl chloride and tetrachloromethane. It is therefore not always economically expedient to produce large amounts of chlorine as a byproduct of the electrolysis
Already known from the 1940's is a process in which alkali-metal sulfate is subjected to electrolysis in order to produce alkali-metal amalgam without the formation of chlorine. The alkali-metal sulfate is electrolyzed between mercury and lead with silver added in an electrolysis cell which is divided by a porous rubber membrane. The amalgam is produced at the cathode and oxygen and sulfuric acid at the anode. The sulfate solution is fed into the cathode space and forced through the diaphragm. The current efficiency in this process is, however, very low and, in addition, the sulfuric acid produced on the anode side is contaminated with sodium sulfate.
To summarize, it can be stated that, in the production process of amalgam using the conventional chlor-alkali electrolysis,. chlorine is produced as a troublesome by-product and this process is furthermore restricted to alkali chlorides as starting material. The chlorine-free process results only in low current efficiencies and a contamination of the reaction products.